The programmable controller is a unit which processes input information by a processing unit (CPU) in accordance with a sequence program stored in a memory (RAM) by a user and selectively operates equipment connected to output terminals in accordance with the operation result to attain a programmed sequence control. The programming may be constructed by a sequencer language which is easy to use for the user, such as LOAD, AND and OR.
A processing system for executing the sequencer language program by the processing unit (CPU) includes an interpreter system in which a quasi-coded program is executed while it is sequentially decoded, and a compiler system in which the program is translated to a machine language so that it is instantly executed. The prior art programmable controller used one of those systems.
The former system, however, has drawbacks of a long time being required for decoding instructions and low processing speed, and in the latter system the processing unit (CPU) may be allowed to overrun if the translated program is destroyed by an external disturbance such as noise because there is no means for checking overrun.
Since those problems directly relate to the performance and safety of the programmable controller, solutions thereof have been desired.